Method of producing a vibration resistant fastener

ABSTRACT

A vibration resistant threaded fastener such as a set screw and method of forming the fastener wherein the load bearing flank of the thread surface of the screw is provided with a series of serrations. Each serration is formed with an inclined ramp terminating in a buttress forming a skewed angle with the radius of the fastener. The ramp surface is inclined in the direction of tightening the fastener so that the buttress presents a surface tending to act against rotation of the fastener to preclude its loosening. In forming the vibration resistant fastener a pair of flat thread roll dies may be used. Each die is provided with a mating segment near the end of the effective thread forming portion of the die having a configuration designed to impart the ramp and buttress serrations on the load bearing flank of the fastener rolled between the dies.

This is a continuation, of application Ser. No. 701,783, filed July 1,1976, abandoned.

The present invention relates to vibration resistant threaded fastenersand more particularly to vibration resistant screws such as set screwssuch as described in commonly assigned U.S. Pat. Nos. 3,972,359,3,972,360 and 3,972,361.

In many applications utilizing a set screw, the set screw is threadablyengaged within one workpiece to have an end bear against andfrictionally grip a second workpiece to secure the two workpiecestogether. In such an application it is imperative, especially where theworkpieces are subjected to vibration, to insure that the set screw doesnot loosen so that the workpieces can move relative to each other.Accordingly, set screws have been designed with anti-rotation featuresin an attempt to minimize loosening or back-off of the set screw whenthe workpieces in which it is installed are subjected to vibration. Onesuggested solution is to provide the seating end or point of the setscrew with an irregular configuration, for example a knurled cupconfiguration, so that the knurled end of the set screw achieves adegree of purchase when it bears against the surface of the workpiece.However, the surface area of the set screw end is necessarily limited sothat the frictional engaging surface is also limited and such set screwshave not provided an entirely satisfactory rotation resistant feature.

Accordingly, it is an object of the present invention to provide arotation resistant threaded fastener such as a set screw which hasnoticeably improved rotation resistant properties when installed in aworkpiece and a method for making such a fastener.

It is a still further object of the present invention to provide arotation resistant threaded fastener with improved rotation resistantproperties and which may be readily and economically manufactured.

It is a still further object of the present invention to provide athreaded fastener which exhibits markedly improved rotation resistantproperties and which may be repeatedly engaged and disengaged from theworkpieces in which it is employed without marked deterioration in therotation resistant property of the fastener.

In accordance with a preferred embodiment of the present invention anirregular surface configuration is imparted to the flank of the threadsof a threaded fastener member which bears the load when the fastener isin compressive loading. The irregular surface configuration on thethread flank is in the form of an inclined ramp and buttress disposed ata skewed angle with respect to a radial of the fastener with the inclineof the ramp being in the direction of rotation to engage the threadedfastener. Thus, once the fastener is fully seated and engaged thebuttress of each of the serrations resists the tendency of the threadedfastener to rotate in the opposite direction and back off.

These and other objects and advantages of the present invention will bemore readily apparent after consideration of the following specificationin conjunction with the drawing.

In the drawing:

FIG. 1 is a schematic representation showing a pair of flat thread rolldies rolling a thread formation on round stock to form the vibrationresistant fastener of the present invention;

FIG. 2 is a plan view of one of the flat thread roll dies showing thethread forming surface including the segment therein to provide aplurality of serrations on one surface of the thread flanks of a screwrolled within the dies;

FIG. 3 is an elevational view taken through the segment to form theserrations on the thread flank of a screw rolled within the dies;

FIG. 4 is a sectional view taken along line 4--4 of FIG. 3;

FIG. 5 is a perspective view showing a preferred embodiment of athreaded fastener according to the present invention;

FIG. 6 is an enlarged perspective view showing one embodiment of theirregular surface configuration placed on the load bearing flank of thethreads of a threaded fastener member according to the presentinvention;

FIG. 7 is a sectional view taken along the helix angle of the threadtaken generally along the line 7--7 of FIG. 6 diagrammatically showingthe orientation of the irregular surface configuration on the loadbearing flank of a thread;

FIG. 8 is a view similar to FIG. 7 showing an alternate embodiment forthe serrations on the flank of the thread;

FIG. 9 is an elevational view taken on the line 9--9 of FIG. 7 with apartial showing of a mating thread configuration;

FIG. 10 is a view similar to FIG. 9 showing an alternate embodiment forthe configuration of the serrations; and

FIG. 11 is an elevational view taken on the line 11--11 of FIG. 9; and

FIG. 11a is an elevational view as in FIG. 11 showing a feature of theinvention.

With reference to the drawing and particularly FIGS. 1 through 4, thereis shown one preferred method of forming a vibration resistant fasteneraccording to the present invention. In this method a round stock member10 is rolled under pressure between reciprocating dies 12 and 14 toimpart a thread configuration and the desired ramp and buttressserrations on the load bearing flank of the formed screw member. Itshould be understood that circular thread roll dies could be utilizedrather than reciprocating dies and that only the latter are disclosed tofacilitate an understanding of the invention.

Dies 12 and 14 are rectangular blocks provided with thread-formingsurfaces 16 and 18, respectively, on one face of each of the dies. Thethread forming surfaces comprise a series of parallel ribs 20 disposedat an angle to the longitudinal axis of the die corresponding to thelead angle of the thread formation to be formed on round stock member10. Each rib 20 has two inclined surfaces 22 and 24 terminating in acrest 26 to form a conventional helical screw thread on the stockmaterial 10.

To form a screw thread formation on stock material 10 the stock materialis placed between the dies 12 and 14 in contact with the thread formingsurfaces 16 and 18 adjacent the lead ends 28 and 30, respectively, ofthe dies. A compressive force is placed on the dies and either one orboth of the dies are moved laterally to hold the stock material 10therebetween and form a screw formation on the stock material 10 as thestock material 10 rotates between the dies.

The thread rolling operation is continued until the stock material 10 isreleased at the discharge ends 32 and 34, respectively, of the dies. Tofacilitate removal of the stock material 10 after the screw formationhas been impressed thereon, one of the dies 12 is provided with anoutwardly inclined ramp 36 which facilitates removal of the stockmaterial 10 from the thread rolling operation.

The ramp and buttress serrations are imparted to the load bearing flankof the threaded fastener by complementary serrations formed within arectangular shaped segment 38 of each die block 12 and 14. Theserrations may be formed on each rib 20' within segment 38 prior tohardening of the die block or may be formed by cutting out a rectangularshaped opening in the die block and inserting a plurality of individualrib segments therein and bonding the individual rib segments within thedie block.

Each of the rib segments 20' also includes flanks 22 and 24 terminatingon a crest 26 and each are provided with a series of laterally offsetsteps 40. Each step comprises an inclined portion 42 terminating in anedge 44 which steps down forming a buttress 46. At the base of thebuttress 46 the next adjacent incline 44 begins so that the series iscontinuous to form an inclined buttress and adjacent inclined ramp. Bothinclined portion 42 and edge 44 form a skewed angle with crest 26 forpurposes which will be made clear hereinafter.

As the stock material 10 is rolled into contact with the segments 38 onthe dies 12 and 14, the major thread formation operation has beencompleted. Continued rotation of the stock material 10 under pressurewithin the segments 38 on dies 12 and 14 causes the material on oneflank of threads formed on the stock material 10 to be further worked bythe serrated steps on the rib sections 20' to form a serrated face onthe thread flank having a configuration as will be more fully describedhereinbelow. Because of the skewed relationship between inclined portion42 and edge 44 with crest 26, clearance is provided between steps 40 andthe serrations formed on the threaded flank which allows the steps todisengage from the serrations as the stock continues rotating. Thus, theskewed relationship functions in a manner somewhat similar to thepressure angle formed on involute gears. Without the skewedrelationship, steps 40 would tear the serrations formed on the threadedflank so that the desired form of the latter would be significantlyaltered.

With reference now to FIGS. 5 through 11 of the drawing, the threadedfastener illustrated is a socket set screw 50 constructed in accordancewith the present invention and, as shown, includes a standard threadconfiguration 52 formed in a thread rolling operation. One end 54 of thescrew 50 is provided with a suitable indentation 56 to receive awrenching member to permit the set screw 50 to be rotated intoengagement in a cooperating threaded bore in a workpiece.

As best seen in FIG. 7, the load bearing flank 58 of each individualthread 60, or if desired on the load bearing flanks of at least twoadjacent threads, is provided with an irregular serrated configuration62 around the circumference of the thread to impart a rotation resistantproperty to the set screw 50 when it is engaged in a threaded receivingbore and a compressive load is impressed on the screw.

When the serrations 62 are viewed along a cylinder concentric with thelongitudinal axis of the screw, the serrations are seen to be in theform of teeth. These teeth are generally saw-tooth in shape and eachinclude a vertical wall or buttress 64 extending between the crest 66 ofa given tooth and the root 68 of an adjacent tooth. An incline surfaceor ramp 70 extends between the crest 66 and the root 68 of an adjacenttooth. The ramp 70 of each serration is inclined with respect to a planeparallel to the helix angle of the thread formation of the screw 50 atan angle of between 5° and 25°.

As best seen in FIGS. 7 and 8, a plane passing through the wall orbuttress 64 of each serration is skewed with respect to the radialcenter of the screw 50 with the skew angle being in the range of 15° to35° with respect to a radius taken from the radial center of screw 50 tothe individual serration 62.

As best seen in FIG. 11, the height of buttresses 64 is constant alongthe flank 58 but it should be understood that it could vary, as shown inFIG. 11a. Also, as best seen in FIGS. 11 and 11a, serrations 62 extendfrom the crest 74 of thead 60 inwardly of the pitch diameter 72 andterminates adjacent the root of the thread. If fatigue strength of thefastener is not important to its function, serrations 62 could extend tothe root, but where fatigue strength is important, the serrations shouldterminate adjacent the root of the thread so as not to allow the root tobe marred thereby detracting from its fatigue strength.

The ramp 70 of each serration 62 is inclined in the direction ofrotation of the screw 50 when it is rotated towards its engagingposition, i.e. clockwise for a right-hand thread. Thus, during rotationof the screw 50 into engagement within a cooperatively threadedreceiving bore in a first workpiece, and before any compressive load isapplied to the screw 50, the walls or buttresses 64 of each serrationprovide no frictional impedance to the rotational engagement of thescrew 10. Once the screw is seated so that a workpiece contacting end 76contacts a second workpiece, or once a compressive load is induced onthe screw, further inward movement of the screw 50 becomes restricted.Continual application of a seating torque until a predetermined load isreached impresses a compressive load on the screw 50.

As the compressive load increases on set screw 50, the load bearingflank 58 of each of the threads of the screw 50 is forced into moreintimate contact with a mating thread flank in the internally threadedreceiving bore in the workpiece in which the set screw 50 is engaged.This compressive force between the load bearing flanks of the matingthread forms results in a substantially elastic deformation of the loadbearing flank of the internal thread configuration in the workpiece sothat, in essence, the ramp 70 of each serration is depressed into themating load bearing flank of the internally threaded workpiece, shown at78 in FIG. 9, so that there is a slight lapping over of the materialfrom the workpiece, as at 80, against the buttress 64. This slightdeformation in the internal thread configuration of the workpiece as itabuts against the buttress 64 of each of the serrations 62 precludesrotation of the set screw in the opposite direction which would tend toloosen the set screw or back it off from engagement with the secondworkpiece.

While the depression is slight at each serration so that an appropriate"off" torque may be applied to loosen the set screw, the locking actionprovided by the multiplicity of serrations affords sufficientanti-rotation resistance to effectively preclude the set screw frombacking off when the workpieces are subjected to vibration. Because theindividual deformation adjacent each buttress 64 is essentially elasticin nature, no appreciable damage or distortion results to the threadconfiguration on either the screw 50 or the mating internal threadconfiguration of the workpiece in which it is engaged.

In the embodiment shown in FIG. 7, the buttress 64 and crest 66 of eachserration are oriented in the same plane at the desired skewed angle tothe radial center of the screw 50. Skewing the crest and buttress inthis manner provides a greater surface area of contact between thebuttress 64 and the mating flank of the thread formation of the threadedbore in a workpiece in which screw 50 is engaged than does a buttressoriented along a radius of the screw.

It has been noted that the skewed orientation of serrations 62 minimizesthe tendency of the serrations to tear large chips of material from theinternal mating thread as the screw is removed from the workpiece. Whilenot completely understood, it is believed that this results from thecompressive load between the serrations and the mating thread aredistributed over a larger surface than that provided by a radialorientation so that the unit force between buttresses 64 and the matingthread is less than that provided by a radial orientation. Thus, thereis less likelihood of exceeding the elastic limit of the mating threadwhich, of course, means that there is more likelihood that thedeformation of the mating thread is elastic.

In the embodiment shown in FIG. 8, the buttress 64' and crest 66' ofeach serration are also oriented generally skewed to the radial centerof the screw 50'. In this embodiment the crest and buttress are formedin the shape of an arcuate crown or convolute surface. Such aconfiguration for the buttress and crest increases the surface area ofcontact still further, as the arcuate path from the innermost portion ofeach buttress to the outermost portion is slightly greater than theequivalent path of the buttress 64 shown in FIG. 8.

In FIG. 10, a still further embodiment is shown wherein the buttress 64"is inclined rather than vertical as in the embodiment shown in FIG. 6.This embodiment is more fully described in commonly assigned U.S. Pat.No. 3,972,360.

Thus, it is seen that the present invention provides a screw form whichexhibits markedly improved rotation resistant properties withoutdeleterious effects which mar or distort either the thread configurationof the screw itself or the mating thread configuration in a workpiece inwhich it is installed. Thus, the screw may be employed through repeatedcycles without noticeable loss of effectiveness.

While the present invention has been described illustratively as a setscrew, it is to be expressly understood that the invention may beemployed on any type of threaded fastener such as a bolt or screw to beengaged in a threaded bore in a workpiece or to be engaged within a nut.

What is claimed is:
 1. A method of producing a vibration resistantfastener having a shank with at least a portion of its length providedwith an external helical thread configuration including a root, a crestand a flank therebetween having a plurality of skewed ramp and buttressconfiguration serrations thereon, comprising the steps of: providing diemembers having ribs for forming the externalthread on round stockmaterial, each said die member including a segment having serrationsprovided on one side of each said rib within said segment for impartinga complementary irregular serration configuration on the load bearingflank surface of the fastener being produced, each of said serrationsbeing in the form of teeth extending inwardly from the crest producingportion of said ribs to termination inwardly of the pitch diameter at apoint spaced from the root producing portion of said ribs, each of saidteeth comprising an inclined ramp portion extending from a root to acrest thereof with a buttress extending between said crest and the rootof an adjacent serration in a plane generally parallel with thelongitudinal axis of the fastener being produced, the height of saidbuttress between said crest and the root of an adjacent serration beinggreater at said crest producing portion of said ribs than at said pointspaced from said root producing portion of said ribs, and each saidbuttress being oriented at a skewed angle with respect to the crests ofsaid ribs; rolling the round stock material between said die members toform the external helical screw threads on a portion thereof and toimpart the complementary irregular serration configuration on a segmentof the load bearing flank of the fastener being rolled therebetween; andsmoothly disengaging each said skewed angle buttress on the die membersadjacent the crest producing portion of the ribs from the complementaryskewed angle buttress formed on the stock as it continues rotating,without tearing material therefrom.
 2. A method in accordance with claim1 wherein said skewed angle is within the range having lower and upperlimits of about 15° to 35°, respectively.
 3. A method in accordance withclaim 1 wherein said inclined ramp portion of each of said teeth formsan angle of between approximately 5° and 25° with a plane parallel tothe helix angle of the external thread.
 4. A method in accordance withclaim 1 wherein each buttress is formed to be convolute in shape todefine an arcuate path from its innermost end to its outermost end.